1. Field of the Invention
The present invention generally relates to an aperture-coupled antenna, particularly to an aperture-coupled circularly polarized planar antenna.
2. Description of the Related Art
Wireless systems which have to function in several frequency bands are being developed more frequently. Frequently, compact antennas are necessary to keep the setup volume of the antennas small and to allow usage in portable devices.
It is possible to provide a separate antenna for each frequency band to be used. The disadvantage of using separate antennas, however, is that a multiplexer has to be employed. In addition, the area necessary for the antennas increases when using separate antennas.
Receiving from several different wireless transfer systems by a single broadband antenna is problematic since broadband antennas cannot usually be manufactured at low cost in a compact design. If all the relevant systems are to be received by a single broadband antenna, this will not be possible using a small cheap antenna.
A multi-element antenna having a special radiator for every frequency range can be used for receiving several frequency bands. Most antenna concepts known which are suitable for receiving from two or more frequency bands (dual-band concept and/or multiband concept) and which can be used for and/or in patch antennas, such as, for example, integrated inverted-F antennas (IFAs) and planar inverted-F antennas (PIFAs), comprise only a linear polarization. Well-known antenna shapes of this kind are, for example, described in the book “Planar Antennas for Wireless Communications” by Kin-Lu Wong (John Wiley & Sons, Inc., Hoboken, N.J., 2003).
However, it is desirable in particular for mobile applications to use a circular polarization, since in this case the orientation of transmitting and receiving antennas is uncritical, whereas when using linear polarization, the orientation of the antennas has to be selected appropriately.
A series of antennas which may be integrated comprising a circular polarization are known, however many of the geometries which may be integrated comprise essential disadvantages for generating a circular polarization. Exemplarily, nearly squared patches (planar conductive areas) of coaxial feeding have a low impedance bandwidth, as is, for example, described in the dissertation “Untersuchung und Aufbau von Multibandigen Antennen zum Empfang zirkular polarisierter Signale” by U. Wiesman produced in 2002 at the Fraunhofer-Institut für integrierte Schaltungen in Erlangen. The same is true for aperture-coupled patch antennas having a cross-slot which are described in the master paper having the title “Untersuchung zirkular polarisierter Patchantenne mit Aperturkopplung” by A. Popugaev in 2004 for Fraunhofer Institut für integrierte Schaltungen in Erlangen. All in all, it can be stated that the polarization purity in known broadband circularly polarized patch antennas having only one feeding point is low. On the other hand, spiral antennas exhibit great losses.
An overview of aperture-coupled microstrip antennas can be found in the article “A review of aperture coupled microstrip antennas: history, operation, development and applications” by D. M. Pozar, published in May 1996 at the University of Massachusetts at Amherst and is available on the internet under www.ecs.umass.edu/ece/pozar/aperture.pdf. Further information on the topic of broadband patch antennas can be found in the book “Broadband Patch Antennas” by J.-F. Zuercher published in 1995 by the Artech-House Verlag.
In summary, it can be stated that there is no technologically advantageous antenna design which, with good radiation efficiency and sufficient impedance bandwidth, allows circularly polarized waves to be radiated with high orthogonal polarization suppression. In addition, there is no known technologically simple antenna design which can be realized at low cost which, with good efficiency and sufficient bandwidth, allows a circularly polarized electromagnetic wave to be radiated in two different frequency bands.